Electrophotographic technology has been widely used for copiers, printers, printing machines and the like since high-quality images are instantaneously obtained. As electrophotographic photoreceptors (hereinafter optionally referred to as “photoreceptor”) which lie in the core technology of the electrophotographic technology, there have been widely used photoreceptors using organic photoconductive materials having advantages such as non-pollution, ease in film formation and ease in manufacture.
Recently, owing to the requirement for high-quality images, a toner size has been miniaturized and particularly, in a chemical toner, since the shape frequently takes a form close to a sphere, toner-passing-through is prone to occur at the time when a toner remaining on the photoreceptor is cleaned by means of a blade and, as a result, there is a high possibility of causing image defects such as scumming. Therefore, there is frequently taken a countermeasure that the cleaning blade is abutted to the photoreceptor with a strong pressure to prevent the toner from passing through.
When the abutting pressure of the cleaning blade to the photoreceptor increases, the bade causes chattering owing to repeated sticking/slipping on the uppermost surface of the photoreceptor, a so-called stick/slip phenomenon and, as a result, there increases a risk of occurrence of insufficient cleaning and abnormal noise. Moreover, by rotation in a state where an external additive and a toner carrier that are toner components are strongly pressed against the photoreceptor through a nip part by the cleaning blade, a decrease in the life of the photoreceptor life due to increased wear of the photosensitive layer and image defects due to the occurrence of the circumferential scratches are prone to occur. Furthermore, a so-called filming phenomenon that the external additive and wax as toner components are fixed to the surface of the photoreceptor and the removal becomes difficult tends to occur, so that a risk of generation of sustained image defects also increases.
As such, it is desired for the photoreceptor to have surface mechanical properties so as to minimize the image defects, abnormal noise, and the decrease in the life attributable to the filming even under more severe use conditions. As a method for improving the surface mechanical properties, such as filming, a method of improving the surface mechanical properties by providing a protective layer on the uppermost surface layer of the photoreceptor has been investigated. However, the productivity is lowered and the cost increases, so that it is difficult to apply the method in many cases other than high-end machine applications.
On the other hand, the photoreceptor is reduced in diameter on the trend of miniaturization and increase in speed of an electrophotographic device, and more improved electrical response (quick decrease in the surface potential of the photoreceptor after exposure) has been demanded. In order to provide an electrophotographic photoreceptor satisfying the characteristics, it is necessary to develop a highly functional charge transport substance showing high mobility and sufficiently low residual potential at the time of exposure. In order to solve the problems, many studies of charge transport substances in which n-electron system is extended by a styryl group or the like with a triphenylamine skeleton or a tetraphenyl benzidine skeleton (Patent Documents 1 to 6).
Incidentally, a photosensitive layer of the electrophotographic photoreceptor using an organic material is obtained by dissolving a charge transport substance, a binder resin and the like in a coating solvent and applying and drying the resulting coating fluid. The points required for the charge transport substance in manufacturing the electrophotographic photoreceptor are solubility in the coating solvent used at the manufacture of the coating fluid and compatibility with the binder resin. When the solubility and compatibility are low, it may not be able to dissolve a desired amount of the charge transport substance in the coating solvent, and degradation of the coating fluid, such as deposition, is prone to occur after the charge transport substance is dissolved to manufacture the coating fluid. Furthermore, crystals may be precipitated in the coated film after the photosensitive layer is coated, thereby inviting reduction of the manufacturing efficiency of the coating fluid and the photoreceptor.
In general, a compound having an extended π-electron system in the molecule shows increased intermolecular interactions as the molecular size increases and the solubility tends to decrease. The tetraphenylbenzidine skeleton described above has a large molecular size and tends to show low solubility and, when a styryl group or the like is substituted in the tetraphenylbenzidine skeleton to extend the π-electron system in the molecule, the molecular size further increases and the solubility in the coating solvent becomes even lower. In some of the reports previously mentioned, in order to ensure the solubility, there has been made a contrivance of handling as a geometric isomer mixture (Patent Document 6).